At the present time, telecommunication systems are largely based on fiber optic cables. For example, optical networks based on fiber optic cables are currently utilized to transport Internet traffic and traditional telephony information. In such applications, it is frequently necessary to provide an optical signal over significant distances (e.g., hundreds to thousands of kilometers). As optical signals travel through the optical fibers, a portion of their power is transferred to the fiber, scattered, or otherwise lost. Over appreciable distances, the optical signals become significantly attenuated. To address the attenuation, optical signals are amplified. Typical optical amplifiers include rare earth doped amplifiers (e.g., Erbiumdoped fiber amplifiers).
Also, Raman amplifiers may be utilized. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is conserved as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
The probability that a Raman scattering event will occur is dependent on the intensity of the light as well as the wavelength separation between the two photons. The interaction between two optical waves due to SRS is governed by the following set of coupled equations:                     ⅆ                  I          P                            ⅆ        z              =                                        λ            S                                λ            P                          ⁢                  g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          P                ⁢                  I          P                                        ⅆ                  I          S                            ⅆ        z              =                            g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          S                ⁢                  I          S                    where Is is the intensity of the signal light (longer wavelength), Ip is the intensity of the pump light (shorter wavelength), gR is the Raman gain coefficient, λs is the signal wavelength, λp is the pump wavelength, and αs and αp are the fiber attenuation coefficients at the signal and pump wavelengths respectively. The Raman gain coefficient, gR, is dependent on the wavelength difference (λs−λp) as is well known in the art.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have come into existence only over the past few years. These advances have renewed interest in Raman amplifiers.
FIG. 1 depicts an exemplary prior art arrangement of optical system 10 which includes a Raman amplifier. Optical system 10 includes optical signal source 11 which launches an optical signal into optical fiber 12 to be communicated to detector 13. Detector 13 is disposed an appreciable distance away from optical signal source 11. Accordingly, optical amplification is required due to attenuation in optical fiber 12. Raman pump source 14 provides a Raman pump to multiplexer 15. Multiplexer 15 provides the Raman pump to optical fiber 12 to generate the desired optical gain.
A key performance parameter of Raman amplifiers is the gain flatness of the amplifier. Gain flatness can be quantified by measuring the gain ripple (variation in gain experience by the optical channels) across the amplification band. To compensate for non-zero gain ripple, a gain flattening filter may be applied to the optical signal to equalize the gain between channels. However, this is a non-optimal solution, since this approach adds loss and therefore decreases the signal-to-noise (SNR) ratio of the system. This becomes an issue when an optical network comprises multiple spans of fiber. Moreover, it shall be appreciated that Raman amplification may be altered by any number of factors on a time variant basis. To compensate for such differences, a gain flattening filter may be applied to the optical signal to equalize the gain between channels. However, this is a non-optimal solution, since this approach increases overall system loss. This additional loss must be compensated with increased gain which, in turn, increases the ASF noise floor and, hence, decreases the signal-to-noise (SNR) ratio of the system.